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Gem bridge: Back to the future

A new Brunel influenced bridge across a secluded valley in Dartmoor National Park is about to help strengthen transport links with France.

A piece of civil engineering with cross Channel significance has recently been completed at Grenofen in the Dartmoor National Park.

The modest size of the 200m long Gem Bridge across the Walkham Valley belies its grander purpose in forging improved cycling links between the South West of England and France. The structure is on the route of Cycle West’s Vélodyssée – a 440km cross-Channel link that will eventually stretch from Ilfracombe in Devon to Redon in Southern Brittany via Okehampton, Tavistock and Plymouth.

By this autumn, the full 26km long stretch of the Drake’s Trail cycle route from Tavistock to Plymouth will fully open. This is where the new cycle and pedestrian bridge plays a vital role.

Project client Devon County Council has placed a great deal of emphasis on striking a perfect balance between elegant design, buildability and a £2.1M budget.

“One thing it always comes back to is the original Brunel structure and trying to tie ours back to the historic one”

Ben Naylor, Devon CC

For the Engineering Design Group at Devon County Council the project was a rare departure from its more typical standard highway bridges work, and the team embraced the challenge with gusto.

“From the start, buildability was key,” says project supervisor Ben Naylor, adding that this was matched by a strong desire to produce a structure that is so aesthetically pleasing that it will suitably flatter its surroundings. A few factors increased the pressure here – one being the level of interest from stakeholders such as the Dartmoor

National Park Authority, and another, not insignificant consideration was that Brunel had been responsible for the viaduct’s predecessor.

The alignment of the new viaduct hugs that of the original Walkham Viaduct, which Brunel designed for the South Devon Railway. This high level viaduct incorporated timber columns fanning upwards from its masonry piers and was built in 1859. It was replaced by a steel lattice truss in 1910 but the structure was demolished in 1965 following the closure of the railway.

Cross Channel Collaboration

The bridge was funded by Devon County Council to the tune of £1.5M but this was supplemented by £600,000 from the EU CYCLE project within the European Regional Development Fund’s Interreg 2 programme.

Devon County Council and the Brittany Tourist Board have collaborated on a project involving Cornwall, Dorset and 14 councils in France to develop a range of cycle routes crossing the English Channel. The council says that without the cross-Channel scheme helping with funding the Gem Bridge would not yet exist.

A variety of funding streams have been used over the years to support the development of the approach routes and bridge design including the now abolished South West Regional Development Agency, other sources of European funding, money from the sale of Exeter airport and a “dowry” from British Rail, Network Rail’s predecessor.

The original viaduct was one of the largest railway bridges in Devon with a total length in excess of 300m, a height above the river of around 40m and 16 truss girder sections supported on masonry piers.

The sense of the site’s history began to take on an increasing significance as design options for the new bridge were narrowed, with the influence of Brunel’s design increasingly coming to the fore.

A cable-stayed structure was ruled out for fear that the structure’s height would be too dominant amid the tree tops and the valley’s intimate setting. A bridge that required its deck to be launched, was also ruled out thanks to the site constraints. This led the team designing a less imposing bridge that could be lifted into place. Then the focus centred on what would look great.

“We could have designed a flat bottomed truss,” says Naylor. “But it was important that in designing the different elements we tried to keep the same ethos that was behind Brunel’s structure. You look for influence wherever you can find it.

“One thing it always comes back to is the original Brunel structure and trying to tie ours back to that historic one,” he says. The team was also keen to take influences from elements of the 1910 structure.

“Why not echo the work of great engineers of our time as well as trying to keep it right for the environment”

Ben Naylor, Devon CC

The result is an intricately designed bridge that incorporates elegant, softly textured concrete piers, a precast concrete deck and a painted, arching steel lattice truss that in ways mimics Brunel’s timber fans.

While an elegant design was key, so was keeping it subtle to make sure it blends in well with the surroundings. The truss, along with the bridge’s specially designed 1.4m tall steel parapets, have been painted dove grey, which, along with the textured concrete, aim to do just that.

Challenges for the designers included limited site access and steep sided embankments, not to mention the fact that mine-shafts up to 50m deep are scattered around the site.

As the bridge is part of a cycle and pedestrian route it connects the two sides of the valley at different heights, so the design had to be managed to ensure a maximum gradient of 1 in 20.

The five spans – in sequence measuring 30m, 40m, 60m, 40m, and 30m – are formed from 15, 15m long steel truss sections fabricated by Tema Fabrication in Cardiff, equating to 150t of steel. Each section had to fit with just 5mm tolerance and its components were bolted together on the valley floor before being lifted into place.

Elegant piers

Each of the four cast insitu piers was designed to fit with the same principle of elegance. All are different heights - up to a maximum of around 22m - and are rectangular in plan with longest sides varying in length from 3m at mid-height to 3.5m at the top and bottom. The other two sides are 1m across for the full height (see drawing). The tallest of the piers required an 11-storey scaffold to install the steel reinforcement, place the shutters and pour the concrete.

In addition to the tricky access, the project’s National Park location influenced the client’s decision to use a design and build contract, despite the fact that Naylor and his team remained in control of design. This was in part because the number of potential variables that could affect the build would have meant that the contractor would have been unable to take the risks without such a contract.

The contract was awarded to Dawnus Construction, which began work on site in March 2011.

Crane companies were engaged early so they could understand the access constraints and reassure the Devon County Council engineering team that the needs of the job could be satisfied when it came to lifting the steel truss sections and the precast concrete deck sections.

The project was helped by site workers grading some of the embankment to ensure the cranes could get in to position at the bottom of the steeply sloping valley. The height of the piers and truss section goes to a maximum height of 25m at mid-span. The southern span was the first to be installed, followed by the two north spans and then the central sections.

Crane lifts

Cranes were used to install the 167 precast concrete deck units, totalling 380t. The deck creates a generous 3.5m wide cycle and pedestrian pathway, with extended width at four viewing points created to enable users to enjoy the views across the valley.

It’s clear that the design is something the team is pleased with, regardless of whether opinion is divided on how much the Brunel influence comes through. “You could argue that it’s not exactly like [Brunel’s],” says Naylor. “But why not echo the work of the great engineers of our time as well as trying to keep it right for the environment.”

In another nod to the past, the remains of the Brunel abutments are still evident. Some of Brunel’s original granite plinths have also been re-used to form picnic and play areas.

The new bridge will be formally opened in the autumn.

Project team

Engineering design Devon County Council Engineering Design Group

Main contractor Dawnus Construction

Main contractor designer Ramboll (appointed as Gifford)

Steel fabricator Tema Engineering

Formwork Sims & Salter

Parapet manufacturer P&R Engineering

Earthworks, geotechnics South West Highways

Geotechnical consultants Frederick Sherrell, Saxton Drilling

 

 

Readers' comments (1)

  • An elegant and sympathetic bridge, I agree. It's such a shame that there wasn't a little more creativity (or budget perhaps) in the handrailing to complement the steelwork beneath. Particularly as many pedetrians and cyclists will stop and spend time leaning on the railings without walking down to view the bridge from underneath.

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